Session D55: Scanning Tunneling Microscopy of Topological Materials

Strain induced semimetal-insulator transition in monolayer 1T’-WTe2

Monolayer 1T'-WTe₂ has exhibited clear quantized transportation originated from the quantum spin Hall phase, while the channel length with quantized conductance is restricted around 100 nm due to its semimatellic nature. To overcome this problem, it is crucial to find a way to engineer the electronic structure. Through a combined in-situ scanning tunneling microscopy/spectroscopy (STM/STS) and Density functional theory (DFT) study, we demonstrate that strain can effectively manipulate the band structure of monolayer 1T'-WTe₂ to realize a phase transition from a semimetal to an insulator upon application of a compressive strain along a axis or a tensile strain along b axis. Moreover, the topological edge states are found to be very robust against strains. Besides, we find that the out-of-plane distortions can also significantly change the band structure. Our results reveal the clear strain dependence of electronic properties and phases of monolayer 1T'-WTe₂, which provides an effective approach to realize the long-coherent quantized conductance for the further transport experiments verification.   

Finite-temperature spectroscopy in dirty helical Luttinger liquids

I will discuss a theory of finite-temperature momentum-resolved tunneling spectroscopy (MRTS) for disordered two-dimensional INTERACTING topological-insulator edges. The MRTS setup measures the spectral properties and THUS provides a complementary way (distinct from transport) to characterize helical Luttinger liquid edges of topological insulators. The (exact within bosonization) finite-temperature spectral function and the tunneling current of MRTS are derived in the presence of disorder and interaction. The theory provides a detailed analytical characterization of MRTS between helical edges, that should be of interest for corresponding experimental studies.   

Atomically resolved study of the effect of back-gating on the band structure in monolayer WTe2

Two-dimensional topological materials have generated a lot of excitement as systems potentially exhibiting quantum spin Hall (QSH) effect, ballistic 1D transport, and helical edge modes. Monolayer 1T'-WTe₂ has been demonstrated to be both a QSH insulator and a superconductor upon doping using electronic transport and photoemission spectroscopy on single crystal flakes and epitaxial films. Atomically-resolved scanning tunneling microscopy (STM) also demonstrated clear edge states on WTe₂ monolayer islands. Here, we report the first STM study of monolayer 1T'-WTe₂ with in-situ tuning of carrier concentration using back-gating. We demonstrate band structure changes in response to doping and back-gating and use first principle calculations to explain the observed effects.   

Resolving the topological classification of bismuth with topological defects

Bulk boundary correspondence has been the cornerstone in the study of topological quantum materials. It has enabled the exploration of electronic bulk properties through the investigation of topological boundary modes. However, the growing diversity and profusion of topological classes has lead to ambiguity between classes sharing similar boundary phenomenology. This is the current status of bismuth, for which recent studies have suggested nontrivial classifications like strong or higher-order TI, both of which hosts 1D helical modes on their boundaries. Here, we use a novel approach to resolve the topological classification of bismuth by spectroscopically mapping the response of a topological lattice defect like screw dislocation using a scanning tunneling microscope. We find a 1D edge mode, bound to the step edges of bismuth, extending to the core of the screw dislocation without gapping out. This signifies that the edge mode binds to the topological defect, characteristic of a material with nonzero weak indices. This work paves the way for the identification of novel electronic topological phases through the study of boundary modes associated with topological defects.   

Domain walls as possible realization of edge state coupling in a quantum spin Hall insulator

The recently discovered monolayer system bismuthene/SiC(0001) is a promising candidate for the realization of a room temperature quantum spin Hall (QSH) effect. Previous experiments have established a large fundamental band gap (0.8 eV) and the existence of one-dimensional metallic edge states [1]. As expected for a QSH insulator, the electronic edge channels do not show any signs of backscattering from kinky edge sections that would manifest in interference phenomena. Notwithstanding, topological protection against defect scattering may become lifted when two helical edge channels are brought into direct proximity, resulting in quantum interference. By scanning tunneling microscopy we study phase-slip domain boundaries (DB) that form as result of bismuthene being a rt3 x rt3 R30° reconstruction on SiC(0001) as substrate. Kinks and disorder limit the longitudinal extent of these quasi one-dimensional topographic defects. By spectroscopic means we scrutinize quasi-particle interference along the DB that points towards a linear electronic dispersion strongly reminiscent of a Fabry-Pérot resonator. We discuss our findings as possible quantum interference between coupled helical edge states that are formed in the vicinity of a DB.

[1] F. Reis et al., Science 357, 287 (2017)   

Drumhead surface state in ZrSiTe probed by scanning tunneling microscopy

The family of materials ZrSiX (X = S, Se, Te) are topological nodal-line semimetals characterized by linear band crossings in 1-dimensional lines or loops in momentum space, rather than discrete points as in Dirac or Weyl semimetals. ZrSiTe has a non-symmorphic crystal symmetry, protecting nodal lines at high symmetry lines in the Brillouin zone (BZ). ZrSiTe is host to four nodal lines, two of which form loops in the BZ, one encircling gamma and the other encircling Z, and the other two forming lines that extend through the BZ. It was theoretically predicted that in the surface projection of ZrSiTe, the area between the nodal loops would contain a drumhead state, an exotic, topologically protected 2-dimensional surface state that links the nodal loops together [1]. Surface states have long been probed by scanning tunneling microscopy (STM) using quasiparticle interference (QPI) measurements, and have also been able to provide additional information on the topological nature of these states. Here, we show the first observed signature of electronic scattering within the drumhead state using low-temperature STM and QPI measurements.

[1] Muechler et al., arXiv:1909.02154   

Symmetry dictated grain boundary state in a two-dimensional topological insulator MoTe2

Structural imperfections such as grain boundaries (GBs) and dislocations are ubiquitous in solids and have been of central importance in understanding the nature of polycrystals. In addition to their classical roles, the advent of topological insulators (TIs) offers a chance to realize distinct topological states bound to them. Although dislocations inside three-dimensional TIs are one of the prime candidates to look for, their direct detection and characterization is challenging. Instead, in two-dimensional (2D) TIs, their creations and measurements are easier and, moreover, topological states at the GBs or dislocations are intimately related to their lattice symmetry. However, such roles of crystalline symmetries of GBs in 2D TIs have not been definitively measured yet. Here, we present the first direct evidence of a symmetry-enforced Dirac type metallic state along a GB in 1T’-MoTe₂, a prototypical 2D TI. Using scanning tunneling microscopy, we show a metallic state along a grain boundary with non-symmorphic lattice symmetry and its absence along another boundary with symmorphic symmetry. Our large-scale atomistic simulations demonstrate hourglass like nodal-line semimetallic in-gap states for the former, whereas the gap opens for the latter, explaining our observation well.   

Scanning Tunneling Microscopy Imaging of Electronic Waves on the Surface of Molecular Beam Epitaxy Grown Ag2Se

Recently, silver chalcogenide systems are being studied as possible candidates for topological insulators (TIs). While experimental works have been reported only on the electronic transport of anisotropic Dirac fermions in silver chalcogenides, no local scanning tunneling microscopy (STM) studies have been done on these materials. Here, we report the synthesis of epitaxial Ag₂Se nanostructures using molecular beam epitaxy (MBE) method. Based on atomic-resolved STM images, a new monoclinic structure is proposed for the MBE grown Ag₂Se, which was not seen in this system before. The electronic structures of the Ag₂Se have been investigated by means of scanning tunneling spectroscopy (STS). To test the unique feature of topological surface states (TSSs), we conducted a low-temperature STM/S study on the surface states of Ag₂Se films. On the selenium (Se)-terminated surfaces, evidence for TSSs are observed in the quasi-particle interference patterns. The existence of standing waves strongly supports the surface nature of topological states. Our results may help resolve the current controversy on the topological nature of Ag₂Se.   

Perfect Transmission of Topological Surface States Through Surface Barriers on Sb(111)*

Topological surface states are unconventional two-dimensional electronic states that are protected from backscattering and overcome surface barriers. Previous experiments have shown that topological surface states have an enhanced transmission and a reduced reflection at the common bilayer step edges on the (111) surface of topological semimetal antimony (Sb), indicating the presence of backscattering against bilayer step edges. We use scanning tunneling microscopy and spectroscopy to investigate Sb(111) surface, which exhibits novel monolayer step edges. We demonstrate that topological surface states transmit through these monolayer step edges perfectly without backscattering. Furthermore, we discover localized states on these edges resembling the helical edge states originating from higher-order topology.   

Quasi-particle interference and confinement effects in a correlated Rashba spin-orbit split 2D electron gas

Rashba spin-splitting enables manipulation of spins with electric fields, opening avenues to transformative device concepts. In most materials where Rashba-spin splitting has been observed, the underlying electronic structure is uncorrelated and therefore well described by electronic structure calculations. This is not the case for correlated systems, in which electronic repulsion is needed to be accounted for. Here, we report a scanning tunnelling microscopy, quasiparticle interference (QPI) study of the two-dimensional electron gas (2DEG) at the surface of PdCoO₂, a correlated oxide system reported to exhibit giant Rashba-like spin-splitting [1] Our QPI data reveal a complex quasiparticle scattering pattern which, in particular, consists of a rounded-hexagon shaped, hole-like scattering band that disperses with averaged effective masses of ~ -13.0 m_e and ~ -11.1 m_e along the G-K and G-M directions, respectively. Through comparison with tight-binding calculations, we also show that the scattering is well described once the spin-selection rules are accounted for. Observation of quantized states in a 5-nm wide terrace region confined by a set of parallel step-edges is also discussed.   

Observation of backscattering induced by magnetism in the topological hinge state of bismuth

We have investigated the effects of time-reversal symmetry breaking on the topological hinge state of bismuth. Using spectroscopic imaging and spin-polarized measurements with an STM, we have compared quasiparticle interference (QPI) occurring in the hinge state of a pristine bismuth bilayer with that occurring in the hinge state of a bilayer, which is terminated by ferromagnetic iron clusters. Our experiments on the decorated bilayer edge reveal an additional QPI branch that can be associated with spin-flip scattering across the Brioullin zone center between time-reversal band partners. The observed QPI characteristics exactly match with theoretical expectations for a topological edge state having one Kramer's pair of bands and, thus provide spectroscopic evidence for back scattering in the topological edge states of bismuth.